A governor for a mechanical timepiece accurately regulates a rate of the timepiece. The governor includes a balance spring and a balance wheel. The balance spring has been made of metal. However, the balance spring made of silicon has been recently used. The silicon balance spring can be formed by a semiconductor process, which makes the dimensional accuracy of the silicon balance spring more accurate than that of the metal balance spring. However, the silicon balance spring is less durable against impact compared to the metal balance spring. Therefore, a silicon balance spring, the base material of which is applied with a strength enhancing coating such as a diamond-like carbon (DLC), has been known.
However, the balance spring with such a coating has a problem related to temperature characteristics that the change rate of the spring constant relative to temperature increases to deteriorate the accuracy of the rate of the timepiece compared to that of the balance spring with no coating. Deterioration of the temperature characteristics of the balance spring prevents the governor from accurately regulating the rate of the timepiece. Meanwhile, it is also known that a silicon balance spring with a coating such as a silicon dioxide (SiO2) coating improves the strength of the balance spring, and also improves the temperature characteristics of the balance spring (see Patent Literature 1: JP 3154091 U and Patent Literature 2: JP 4515913 B, for example)